Tracking Back Animal Evolution

Scientists interested in ancient life have a wealth of fossils and impressions frozen in rocks that they can study from as far back as 540 million years ago – when animals with shells and bones began to become plentiful. But evidence of complex life older than 540 million years is scant and difficult to study.

Now, a research team from Virginia Tech in the United States and Nanjing Institute of Geology and Paleontology in China has discovered uniquely well-preserved fossils form from 550 million year old rocks of the Ediacaran Period. Shuhai Xiao, geoscientist from Virginia Tech, with Bing Shen, a Virginia Tech graduate student, and Chuanming Zhou, Guwei Xie, and Xunlai Yuan, all of the Nanjing Instititue of Geology and Paleontology, report in the Proceedings of the National Academy of Sciences (PNAS) the discovery of these unusually preserved fossils which reveal unprecedented information about the body construction of macroscopic organisms more than half billion years ago.

Ediacara fossils, named after Ediacara Hill in the Flinders Ranges of South Australia where such fossils are best known, are some of the oldest fossils with large body size and complex morphology. "Present in rocks ranging from 575 to 540 million years in age, these fossils provide the key to understand the prelude to the Cambrian Explosion when animals with skeletons and familiar morphologies began to bloom about 540-520 million years ago," Xiao said, "However, classic Ediacara fossils are mostly preserved in sandstone, and the coarse sand grains limit how much we can learn about the fine-scale morphologies of these fossils."

Partly because of this limitation, scientists cannot agree on the fine-scale anatomy of Ediacara organisms and have been debating for decades their relationships with animals and other macroscopic life forms. Traditionally, Ediacara organisms are thought to be related to such animals as jellyfishes and worms. Other scientists, however, believe that they may be plants or fungi. Twenty years ago, however, Adolf Seilacher, a paleontologist now retired from University of Tubingen (Universitat Tubingen) and Yale University, argued that many Ediacara organisms were built of tube-like elements and are only distantly related to living animals. "But direct observation of the hypothesized tube-like elements has been difficult because such tubes tend to be deflated and squashed prior to their preservation in sandstones," Xiao said.

This may change with the new discovery of Ediacara fossils from fine-grained limestone of the Dengying Formation in South China by Xiao and his collaborators. "The Ediacara fossils from China were not deflated before they were incorporated in the rock," said Shen, "instead, they are preserved three-dimensionally in the rock." Using serial thin sectioning techniques, Shen and Xiao cut the decimeter-sized fossils into many paper-thin slices and looked at them under a microscope. They saw organic remains of millimeter-sized tubes that were the building blocks of the Ediacara fossils from China. Their discovery thus directly confirms Seilacher’s hypothesis.

The new fossils also help to refine the Seilacher hypothesis. Seilacher originally hypothesized that Ediacara tubes had closed ends and were filled with cytoplasm, or cell contents. The fossils from China, however, appear to have an open end that is connected with the external environment. Thus, Xiao and his colleagues infer that the tubes of their Ediacara fossils were probably not filled with cytoplasm.

Ediacara organisms had no shells or bones. How could such soft and delicate organisms be preserved in rocks? Working with Geosciences Professor Fred Read at Virginia Tech, Geology Professor Guy Narbonne at Queen’s University, and paleontologist James Gehling at South Australia Museum, Xiao and his colleagues carefully examined the calcite crystals that replicate the tubes. They believe that the crystals were emplaced shortly after the death, burial, and decay of the Ediacara organisms, thus replicating the three-dimensional shape of the tubular structures.

How did these Ediacara organisms live? "We think that the fossils were preserved where they lived. In other words, they had not been transported a long way from their deathbed to their graveyard." Zhou said, "And the way they occur in the rocks suggests that they were flat-lying organisms sprawling on the ocean floor 550 million years ago, much like some fungi, lichen, or algae do today."

The resemblance to modern fungi, lichen, or algae may stop at the seemingly similar life position on ocean floor. "In fact, the morphology of the new fossils is unlike any living macroscopic life," Xiao said, "and at this time it is still uncertain how the Ediacara fossils from South China are related to other Ediacara organisms and to living organisms."

But the new fossils surely will enlighten the ongoing debate on the nature of Ediacara organisms that lived just before the evolution of familiar animals about 540-520 million years ago.